1. Field of the Invention
The present invention relates to an image signal recording and reproducing system for recording an image signal on a recording medium and for reproducing a recorded image signal from the recording medium.
2. Description of the Related Art
It has heretofore been known that an image signal recording and reproducing system for recording an image signal on a recording medium and for reproducing a recorded image signal from the recording medium, such as a video tape recorder or an electronic still video camera, has the function of displaying a reproduced image signal on a television monitor or the like as a video image. However, such a system can be used in various other applications. In one application, after a reproduced image signal for one picture is temporarily stored in a memory or the like, the stored image signal may be read out to produce a hardcopy of an image corresponding to the read image signal by means of a printer. In another application, an image signal read from the memory may be transmitted to a remote location by means of a transmission device.
To suppress time base variations which may occur in an image signal during reproduction from a recording medium, such a system is commonly arranged to perform so-called TBC (time base correction) processing when the image signal reproduced from the recording medium is to be stored in the memory. The TBC processing generally includes the steps of forming a clock signal having time base variations equivalent to those of a reproduced image signal, digitizing the reproduced image signal in synchronization with the clock signal, storing the digital image signal in a memory, reading the stored digital image signal from the memory in synchronization with a clock signal having no time base variations, and restoring it to an analog signal.
The above-described TBC processing commonly utilizes either of the following methods to form the clock signal having time base variations equivalent to those of a reproduced image signal.
The first method includes the steps of separating a synchronizing signal from a reproduced image signal and forming, through a PLL (phase-locked loop) circuit, a gated oscillator or the like, a clock signal phase-synchronized with, for example, the falling edge of the separated synchronizing signal.
However, the first method has a number of problems. For example, noise or the like may occur during reproduction, resulting in degradation of a reproduced image signal. Although an image signal is emphasized and then frequency-modulated during recording, if overmodulation occurs in this process, the waveform or phase of an edge portion of a synchronizing signal included in the reproduced image signal is impaired. As a result, it may be impossible to accurately separate the synchronizing signal and hence to effect highly accurate TBC processing.
To cope with the above-described problem, the second method may be used which includes the steps of adding a pilot signal to an image signal in such a way that a pilot signal having a single frequency (for example, approximately 2.5 MHz) is frequency-multiplexed with an intermediate band (near approximately 2.5 MHz) between a luminance signal band (Y-FM) and a chrominance signal band (C-FM) as shown in FIG. 1, and recording the resultant signal on a recording medium. In reproduction, a clock signal is formed by a PLL circuit by using a reproduced pilot signal.
According to the above-described second method, since each pilot signal is continuously added to an image signal to be recorded, it is possible to apply highly accurate TBC processing to time base variations which may occur during reproduction.
However, if a pilot signal is to be frequency-multiplexed with an image signal in the above-described manner, it is necessary to sufficiently consider the frequency and level of the pilot signal to be multiplexed. Otherwise, the image signal suffers an adverse influence such as moire, resulting in degradation of image quality. If a pilot signal is multiplexed with an image signal and recorded on a recording medium at a level which has no adverse influence on the image signal, a pilot signal having a high S/N ratio cannot be obtained during reproduction and highly accurate TBC processing may not be achieved.